South Korea’s nuclear plant operator has said its computer systems have been breached, raising fears that hackers, including those with possible North Korean links, could shift their focus to key infrastructure.

The violation prompted a safety drill on Monday at nuclear plants around the country. The precautionary exercise comes days after the US blamed Pyongyang for hacking Sony Pictures, which led the company to cancel the release of a Hollywood satire about the fictional assassination of the secretive state’s leader, Kim Jong-un.

Officials in the South Korean capital, Seoul, said only noncritical data about nuclear plants had been leaked, adding that they were confident they could fend off any attempt to compromise the safety of the country’s atomic facilities. “It’s our judgment that the control system is designed in such a way – there is no risk whatsoever,” Chung Yang-ho, deputy energy minister, said.

President Park Geun-hye ordered a complete inspection of South Korea’s key national infrastructure against what she called “cyber-terrorism”.

“Nuclear power plants are first-class security installations that directly impact the safety of the people,” Park said at a cabinet meeting, according to her office.

“A grave situation that is unacceptable has developed when there should have been not a trace of lapse as a matter of national security.”

The energy ministry and officials at Korea Hydro and Nuclear Power (KHNP), a nuclear plant operator that is part of the state-run Korea Electric, made no mention of North Korea as a possible suspect.

The latest attack resulted in the leak of personal details of 10,000 KHNP workers, designs and manuals for at least two reactors, electricity flow charts and estimates of radiation exposure among local residents. There was no evidence, however, that the nuclear control systems had been hacked.

“A two-day drill is under way through simulators to ensure the safety of our nuclear power plants under cyber-attacks,” a KHNP spokesman said.

KHNP operates 23 nuclear reactors that provide almost a third of South Korea’s energy needs. “We are making utmost efforts, working closely with the government to assess the data leak at certain nuclear power plants, which adds to social unease,” the company said.

The leaks appeared on Twitter last week via an account claiming to belong to the head of an anti-nuclear group based in Hawaii.

In a message on Sunday, the unidentified hacker threatened to release further information unless the government shut three reactors by Thursday. The message warned people living nearby to avoid the area for the next few months.

The vice-minister for energy, Lee Kwan-sup, confirmed the leaked information appeared to be from the Gori and Wolseong plants south-east of Seoul. The government has been handling this case with extreme care, he said, adding that the leaks posed no safety risk.

Nuclear officials also sought to allay security fears. “It is 100% impossible that a hacker can stop nuclear power plants by attacking them because the control monitoring system is totally independent and closed,” a KHNP official said.

The safety drill came soon after Barack Obama said it had been confirmed that North Korea had hacked Sony Pictures, allegedly because Pyongyang was angered by the portrayal of its supreme leader in The Interview, starring Seth Rogen and James Franco.

Sony Pictures halted the planned Christmas Day release of the film after the hackers threatened 9/11-style attacks on cinemas that screened it.

The FBI cited significant overlap between the attack and other “malicious cyber activity” with direct links to Pyongyang, including an attack on South Korean banks last year.

China, North Korea’s only significant ally, has stopped short of blaming North Korea for recent hacking incidents. Wang Yi, the Chinese foreign minister, had discussed the matter with his US counterpart, John Kerry, the foreign ministry said. Wang had “reaffirmed the country’s relevant position, emphasising China opposes all forms of cyber-attacks and cyberterrorism”, it said.